In mode-locked laser oscillators, the cavity modes get populated during the roundtrips of the light pulses in the oscillator. These oscillators include a mode-locking component that synchronizes the phases of the different cavity modes, organizing them into a series of ultra-short laser pulses.
Several mechanisms can lead to mode-locking, including a temporal or spectral modulation of the amplifying gain, cavity loss, or mode structure of the laser cavity. Broad classes of these mechanisms can be characterized as non-linear optical behavior or response. In actively mode-locked lasers the mode-locking processes are controlled by external active intervention. In passively mode-locked lasers the dynamics of the laser itself modulates the parameters of the cavity. Both classes of lasers can exhibit a rich dynamic behavior that eventually determines the operating parameters and output characteristics of the generated pulses, including the pulse duration, average power, peak power, mode-quality, dynamic stability, and self-starting capability.
The output characteristics of mode-locked laser oscillators usually reflect design compromises between contradicting requirements, as the mode-locking nonlinear behavior is controlling more than one output characteristics of the laser oscillator. An example of such contradicting requirements is the need for the nonlinear optical behavior to be active long enough to self-start the laser oscillator, competing with the need for the nonlinear optical behavior to be active only for a short time to create ultra-short laser pulses.
Some laser designs resolve these contradicting or competing requirements by including two mode-locking components with different nonlinear mechanisms to synchronize the phases. For example, one of the end mirrors or the gain material of the oscillator can exhibit a nonlinear mechanism that effects the phases of the cavity modes, and an additional absorber element can mode-lock the cavity modes by modulating their amplitudes. The above competing design requirements can be resolved by implementing nonlinear elements that have different characteristic time scales.
Another pair of contradictory design requirements involves the beam intensity for the onset of the nonlinear optical behavior. The self-starting functionality prefers low onset beam intensities, whereas an efficient way to shape the pulses into ultra-short pulses involves high onset beam intensities. Again, some lasers include different components relying on different nonlinear mechanisms to simultaneously satisfy these design requirements.
Using two nonlinear elements to resolve the design conflicts in these lasers, however, also introduces problems. Each nonlinear element can be complex and expensive. Moreover, often the benefits come at a cost. For example, the phase-modulating Kerr-effect some designs rely on introduces an unintended and undesirable complex coupling between temporal and spatial effects, modulating the pulse as it propagates through the Kerr-cell-based nonlinear element. This modulation needs to be taken into account when designing the cavity, inconveniently increasing the complexity of the design. Further, the cavity needs to be stable without taking into account the Kerr-effect for the self-starting, and with the Kerr-effect for generating short mode-locked pulses. Fulfilling these requirements is not always possible. The operating range of such designs can be very narrow in terms of tolerances of the cavity parameters. Also, the resulting laser may not be stable against environmental perturbations. Other nonlinear elements can come with comparable challenges as well.